1. Field of Invention
The present invention pertains to the field of magnetic memories. More particularly, this invention relates to a magnetic memory cell with substantially symmetric switching characteristics.
2. Art Background
A magnetic memory such as a magnetic random access memory (MRAM) typically includes an array of magnetic memory cells. Each magnetic memory cell usually includes a sense layer and a reference layer. The sense layer is usually a layer of magnetic material that stores magnetization patterns in orientations that may be altered by the application of magnetic switching fields. The reference layer is usually a layer of magnetic material in which the magnetization is fixed or "pinned" in a particular direction.
The logic state of a magnetic memory cell typically depends on its resistance to electrical current flow. The resistance of a magnetic memory cell usually depends on the relative orientations of magnetization in its sense and reference layers. A magnetic memory cell is typically in a low resistance state if the overall orientation of magnetization in its sense layer is parallel to the orientation of magnetization in its reference layer. In contrast, a magnetic memory cell is typically in a high resistance state if the overall orientation of magnetization in its sense layer is anti-parallel to the orientation of magnetization in its reference layer.
Such a magnetic memory cell is usually written to a desired logic state by applying magnetic switching fields that rotate the orientation of magnetization in the sense layer. It is usually desirable to form a magnetic memory cell so that a magnetic switching field of a predictable magnitude in one direction switches it to its low resistance state and a magnetic switching field of a predictable magnitude in the opposite direction switches it to its high resistance state. Such switching behavior may be referred to as symmetric switching characteristics. Unfortunately, a variety of effects commonly found in prior magnetic memory cells create asymmetric switching characteristics.
For example, the reference layer in a typical prior magnetic memory cell generates demagnetization fields that push the magnetization in the sense layer toward the anti-parallel orientation. These demagnetization fields usually increase the threshold magnitude of the magnetic switching field needed to rotate the sense layer to the low resistance state and decrease the threshold magnitude of the magnetic switching field needed to rotate the sense layer to the high resistance state. This typically increases the power needed to write the magnetic memory cell to the low resistance state and may cause accidental writing to the high resistance state. In extreme cases, these demagnetization fields may cause a magnetic memory cell to remain in the high resistance state regardless of the applied magnetic switching fields.
In addition, coupling fields between the reference layer and the sense layer in a typical prior magnetic memory cell push the magnetization in the sense layer toward the parallel orientation. These coupling fields usually increase the power needed to write the magnetic memory cell to the high resistance state and may cause accidental writing to the low resistance state. In extreme cases, these coupling fields may cause a magnetic memory cell to remain in the low resistance state regardless of the applied magnetic switching fields.
Moreover, the degree of disruption to sense layer magnetization from demagnetization and coupling fields may vary among the magnetic memory cells in an MRAM array and may vary between different MRAM arrays due to variation in the patterning steps and/or deposition steps of device manufacture. Such variations typically produce uncertainty as to the behavior of individual magnetic memory cells during write operations.